Roads, e.g. national highways and motorways, run through landscape with a certain topography which may result in a different gradient α on each section of road. Being able to calculate this gradient α and thereby have access to a relevant value for it is important in various applications in a motor vehicle. An example of such an application is for automatic gear choice, i.e. an application in an automatically operated manual gearbox, where it is important to be able to determine current running resistance and therefore which gear to choose at a given time. Another example of an application is a cruise control which takes the road gradient α into account when determining what torque to demand from an engine of the vehicle. The road gradient α may also be used for example in brake systems and other driver assistance systems.
Estimating the road gradient α is at present often based on an accelerometer which measures the acceleration in a direction of movement of the vehicle. This is illustrated schematically in FIG. 1, in which a motor vehicle 100 is travelling on a section of road which has a gradient α. The vehicle has an acceleration av, and an accelerometer 101 in the vehicle measures the acceleration as, which is the acceleration in the horizontal direction. The accelerometer measures and thus produces a signal corresponding toas=av+g sin(α)  (eq. 1)in which g is the acceleration due to gravity.
This signal can then be used to determine the road gradient α. At small values of α, sin(α) may be approximated to α, which means that the road gradient α may be determined as
                    α        =                                            a              s                        -                          a              v                                g                                    (                  eq          .                                          ⁢          2                )            
The vehicle's acceleration av is thus here subtracted from the as value measured by the accelerometer in order to leave only the gravitational component of the measured acceleration.
This procedure for determining the road gradient α works on well on roads where the gradient α and the curvature are slight. On substantially level and straight roads, e.g. motorways, it produces a relatively good estimate of the gradient α, but is far from optimum on roads and sections of roads which are not substantially level and straight.
When a vehicle travels on a road which does not have only slight gradients a and bends, as on minor roads and certain national highways, the accelerometer will not only measure the acceleration, which is important for determining the gradient α, but will also measure other accelerations which are due inter alia to the road's curvature. These other accelerations, which will thus also be included in the signal as produced by the accelerometer 101, will then adversely affect the reliability of estimation of the road gradient α.